1. Field of the Invention
This invention relates to the preparation of discrete, finely divided, tack-free elastomer particles. More particularly, this invention relates to a process for preparing such particles from solution of an elastomer in a organic solvent.
2. Description of the Prior Art
The processing of thermosetting elastomers into shaped objects is normally a multistage procedure. In contrast to the manufacture of molded components from thermoplastic synthetic resins, it is absolutely necessary, prior to the final shaping step, to conduct generally complicated mixing processes to obtain homogeneous incorporation of diverse additives prior to the cross-linking reaction. The relatively high viscosity of the raw materials makes it necessary to utilize heavy machines, e.g. rolling mills or internal mixers, which perforce require a high energy consumption. The inherent considerable costs of such energy and equipment greatly impair the economy of the production of elastomeric materials.
The statistical distribution homogeneity required of the various components in the elastomer mixtures necessitates, in addition to high equipment investment costs, considerable expenditures in time and energy. During solid state homogenization, the rubber particles and the filler particles are displaced against a strong resistance with respect to one another. Even those portions of the mixture which are sufficiently homogeneous must be further subjected to the mixing procedure until a satisfactory dispersion of the entire mixture has been achieved. The frictional heat generated requires a discontinuous mode of operation. In order to minimize the danger of premature vulcanization, at least a portion of the vulcanizing agents are not added until a second operating step. However, the preparation of crosslinkable elastomers sufficiently stable for even intermediate storage periods requires expensive measures and is effected, in modern large-scale plants, by expensive pelletizing and storage in rotating containers. Further processing involves the subsequent preparation of a blank on calenders and/or extruders, depending on the type of the finished article. The conventional production method is then completed by vulcanization in presses or autoclaves.
Due to the characteristic properties of the raw elastomer material, severe limitations have been encountered in numerous attempts to devise economical, and especially automated manufacturing methods. In order to simplify the aforementioned operating steps, the masterbatches comprising coprecipitates of aqueous elastomer emulsions with carbon black have been proposed, by means of which other mixture components can be incorporated. However, due to their strong inherent tackiness, such raw elastomer mixtures are commercially prepared as bales of hard, solid consistency analogous to the filler-free polymers. Consequently, in spite of cost savings in producing the basic mixture, the machinery traditionally utilized for further processing operations could not be eliminated.
In another attempt to simplify production of elastomer-filler mixtures, a specific precipitation technique was developed to manufacture coprecipitates from synthetic rubber latices (e.g., SBR emulsion polymers) and highly active silicic acid. In this technique, latices containing anionic emulsifiers are mixed with an alkali silicate solution and precipitated with acid in the presence of electrolyte solutions. Examples of such techniques are described in German Pat. Nos. 1,148,067; 1,204,404; 1,297,858; 1,204,405; 1,215,918; and 1,245,093.
The elastomer mixtures obtained by the above method are very fine powders. It is possible to incorporate further additives into these elastomer mixtures with the use of simple agitator systems and without any substantial plasticizing and mixing procedures, and to utilize such elastomer mixtures directly for processing into rubber articles according to conventional methods, e.g., as taught in German Pat. Nos. 1,148,067 and 1,196,367. However, such coprecipitates are not suitable for broad scale applications, since it has heretofore been difficult if no impossible to produce powdery, carbon-black-containing rubber mixtures in this manner.
Other processes for producing carbon-black-containing rubber mixtures from rubber solutions are known, e.g. the so-called "Hydro-Solution-Masterbatch Method" taught in U.S. Pat. No. 2,769,795; Belgian Pat. Nos. 619,992 and 620,801; and French Pat. Nos. 1,440,470, 1,443,769, and 1,371,688, but pourable elastomer mixtures in the form of powders or granules are not obtained by this technique.
In view of the above-described difficulties in these prior art techniques, there is a genuine need for economical processes for the production of pourable, powdery, filler-containing elastomer mixtures, especially for carbon-black-containing elastomers widely used in the rubber industry.